Japanese unexamined patent application publication No. JP-A-11-95628 describes a related art image forming apparatus.
In the related art image forming apparatus, there is known a so-called tandem type image forming apparatus in which photosensitive drums corresponding respectively to respective colors of yellow, magenta, cyan and black are disposed in parallel.
In the related art tandem type image forming apparatus, developing rollers are provided so as to confront the photosensitive drums, respectively. Electrostatic latent images are formed on surfaces of the photosensitive drums. When the electrostatic latent images face with the corresponding developing rollers in association with rotation of the photosensitive drums, toner is supplied from the developing rollers to the electrostatic latent images, whereby toner images are formed on the surfaces of the photosensitive drums. The toner images of colors corresponding to the respective photosensitive drums are formed thereon, and the toner images of respective colors are transferred on to a sheet that is conveyed by a belt in a superimposed fashion, then, the formation of the full color image on the sheet is attained.
In order to form a full color image with high quality, the toner images of respective colors need to be transferred on to the sheet without any color registration error. Therefore, in the related art tandem type image forming apparatus, toner images of respective colors referred to as registration marks are formed at the same time on a belt which is disposed to confront all the respective photosensitive drums, and a so-called color registration error correction is implemented in which timings at which toner images of respective colors are formed (timings at which electrostatic latent images are formed on the respective photosensitive drums) are corrected such that intervals between the registration marks lying adjacent to each other become predetermined intervals.
A cover is provided on a housing of the image forming apparatus so as to be opened and closed. The cover is sometimes opened to perform servicing for maintenance (for example, replacement of parts and removal of jammed sheets of paper), and when color images are formed thereafter, there is a possibility that a color registration error attributed to the maintenance servicing occurs. Therefore, a configuration is proposed in which a sensor is provided for detecting the opening and closure of the cover (door), and when the sensor detects that the cover has been opened and then closed, a color registration error correction is made to be performed in response to the detection by the sensor. In addition, since the sensor remains out of operation and hence cannot detect the opening and closure of the cover in such a state that the power supply for the image forming apparatus is switched off, there is also proposed a configuration in which a color registration error correction is made to be performed in response to the power supply being switched on.